The invention relates most generally to a pressure measuring device.
Conversion of a pressure into electrical data is useful for indicating and controlling the measure pressure. FIG. 1 shows a sectional view of a conventional pressure measuring device. A disc-shaped insulator 2 is inserted into a cylinder 1. The two end faces of the insulator 2 are formed with concave surfaces 3A and 3B, respectively. Metal foil electrodes 4A and 4B are provided on the concave surfaces 3A and 3B, respectively. Lead wires 5A and 5B connected respectively to the electrodes 4A and 4B which extend outside the device through inside the insulator 2 and a hermetic seal 7 which completely closes a hole 6 formed in the wall of the cylinder 1. A through-hole 8 extending from the end face 3A to the end face 3B is formed in the insulator 2. Pressure measuring diaphragms 9A and 9B are welded to the two end faces of the cylinder 1 so that the assembly of the diaphragms and the cylinder is liquid-tight. The space in the interior of the assembly of the diaphragms and the cylinder is filled with an insulating liquid such as silicone oil. The insulating liquid electrically insulates the electrodes 4A and 4B from the pressure measuring diaphragms 9A and 9B and serves as pressure transmitting means between the inner surfaces of the pressure measuring diaphragms 9A and 9B. The electrodes 4A and 4B and the pressure measuring diaphragms 9A and 9B confront each other through the insulating liquid thus forming a capacitor. Connection plates 12A and 12B have recesses 10A and 10B and through-holes 11A and 11B, respectively. The end faces of the connection plates 12A and 12B, where the recesses 10A and 10B are formed respectively, are placed on the outer surfaces of the pressure measuring diaphragms 9A and 9B, respectively, and the connection plates 12A and 12B are fixedly mounted on the pressure measuring diaphragms and are tightened by bolts 13 and nuts 14.
When different measurement pressures are applied to the device through the through-holes 11A and 11B, the pressure measuring diaphragms are deformed by the difference between the measurement pressures and accordingly the capacitance of the capacitor formed by the electrodes 4A and 4B is changed whereby the pressure difference can be detected.
However, the conventional pressure measuring device is disadvantageous in the following points. When the ambient temperature is increased, the insulating liquid expands and accordingly the pressure measuring diaphragms 9A and 9B are pushed outwardly so that the capacitance of the capacitor formed by the electrodes 4A and 4B is changed as a result of which a measured pressure value includes an error. When the ambient temperature greatly decreases so as to significantly contract the volume of the insulating liquid, the pressure measuring diaphragms cannot deform properly following the contraction of the insulating liquid. As a result, a vacuum space called "Torricelli's vacuum" is formed in the insulating liquid making it impossible for the insulating liquid to transmit the pressure correctly so that the pressure is again measured erroneously.
Generally, the cylinder 1 has been made of stainless steel, while the pressure measuring diaphragms 9A and 9B have been of material high in elasticity. That is, the cylinder 1 has a different temperature expansion coefficient from that of the pressure measuring diaphragms 9A and 9B. Accordingly, the amount of radial expansion or contraction of the peripheral portions of the pressure measuring diaphragms 9A and 9B is different from that of the two end faces of the cylinder 1 on which the pressure measuring diaphragms 9A and 9B are mounted. Therefore, the force which pulls the pressure measuring diaphragms radially is changed resulting in an error in the measured pressure.
The connection plates 12A and 12B are mounted by means of the bolts 13 and the nuts 14 in order to apply pressure to the pressure measuring diaphragms 9A and 9B, as described above. However, this causes yet another problem. That is, the cylinder 1 is contracted axially by the tightening force of the bolts 13 and the nuts 14 so that the distances between the electrode 4A and the diaphragm 9A and between the electrode 4B and 9B are changed which induces thereby an error in the pressure measurement.
Accordingly, an object of this invention is to provide a pressure measuring device having a high measurement accuracy in which all of the above-described difficulties accompanying a conventional pressure measuring device have been eliminated.